1. Field of the Invention
This invention relates to a rotary rock bit suitable for drilling in extremely hard formations and, more particularly, to a rock bit having a plurality of conical cutters, such as the well known tri-cone bit, with each conical cutter being independently true rolling on a bottom formed by its cutting structure which is distinct from the bottom formed by the cutting structure of the remaining cutters.
2. Description of the Prior Art
Rotary rock bits, particularly tri-cone bits are well known in the earth boring art for drilling oil and gas wells and for drilling blast holes for ore mining operations.
Generally, such bits include a bit body having a plurality of downwardly extending legs (i.e. one for each cutter) terminating in radially inwardly and axially downwardly extending bearing pin on which the conical cutter is mounted. As the earth through which the bit must bore has varying hardness, etc., the bits are custom designed to exhibit cost effective optimum drilling characteristics in an earth formation of particular hardness. Thus, in rather soft formations, the drilling is accomplished by the conical cutters having relatively long cutting structure extending therefrom. Also, the elastic modulous and strength of the cutting structure relative to the softer earth formations permits the use of an offset in the axis of rotation of the cutter with respect to the bit body (i.e. the axis of rotation of the cutter is not coincident with the axis of rotation of the bit body) or an oversize cone can be employed relative to the borehole diameter. In either instance the cutting structure extending from the cutter is not true rolling, thereby providing a scuffing or gouging action during rotation of the cutter that rapidly disintegrates the earth.
However, for progressively harder formations, to prolong the life of the cutters the cutting structure is shortened and made harder and thus more brittle. For the hardest earth formations the cutting structure is generally made of inserts of tungsten carbide or the like which project for a short distance from the surface of the conical cutter. As this material is relatively easily chipped, side loading thereof is avoided and, to this end, the gouging or scuffing action during drilling is eliminated by having the various axes of the cone and bit coincident and by sizing the defined effective conical exterior to the proper size to provide a true-rolling cutter. Such a cutter disintegrates the earth primarily through compressive fragmentation of the rock formation immediately below the insert as it contacts the formation.
A tri-cone rock bit having true rolling cutters is shown in U.S. Pat. No. 4,056,153 of common assignee to the present invention; however, as will be seen upon review of the above patent, the conical cutters are all of a common size (i.e. the true rolling cone defined by each has a single included angle). Thus, each area of earth contacted by the cutting structure projection has substantial lateral support by the adjacent formation of the borehole bottom in that, because of the equal cone angle, the bottom profile is substantially flat, as shown in FIG. 6 of the patent.